Factors controlling sediment trapping in two freshwater tidal wetlands in the Biesbosch area, The Netherlands

Abstract

Purpose A thorough understanding of mechanisms controlling sedimentation and erosion is vital for a proper assessment of the effectiveness of delta restoration. Only few field-based studies have been undertaken in freshwater tidal wetlands. Furthermore, studies that measured sediment deposition in newly created wetlands are also sparse. This paper aims to identify the factors controlling the sediment trapping of two newly created freshwater tidal wetlands. Materials and methods Two recently re-opened polder areas in the Biesbosch, The Netherlands are used as study area. Field measurements of water levels, flow velocities, and turbidity at both the in-and outlet of the areas were carried out to determine the sediment budgets and trapping efficiencies under varying conditions of river discharge, tide, and wind in the period 2014-2016. Results and discussion Short-term sediment fluxes of the two study areas varied due to river discharge, tide, and wind. A positive sediment budget and trapping efficiency was found for the first study area, which has a continuing supply of river water and sediment. Sediment was lost from the second study area which lies further from the river and had a lower sediment supply. The daily sediment budget is positively related to upstream river discharge, and in general, export takes place during ebb and import during flood. However, strong wind events overrule this pattern, and trapping efficiencies decrease for increasing wind strengths at mid-range river discharges and for the highest river discharges due to increased shear stress. Conclusions Delta restoration, based on sedimentation to compensate for sea-level rise and soil subsidence, could only be effective when there is a sufficient supply of water and sediment. Management to enhance the trapping efficiency of the incoming sediment should focus on directing sufficient river flow into the wetland, ensuring the supply of water and sediment within the system during a tidal cycle, creating sufficiently large residence time of water within the polder areas for sediment settling, and decreasing wave shear stress by the establishment of vegetation or topographic irregularities.

Floodplain Wetland Channel Planform, Cross‐Sectional Morphology, and Sediment Characteristics Along an Estuarine to Tidal River Gradient

“…The most landward wetland we sampled at rkm 228 is presumably directly exposed to effects of the dam, but had a positive accretion rate consistent with its protected position on an inlet enclosed by an island. The potential role of winds in sediment transport and trapping efficiencies (Prokocki et al., 2020; van der Deijl et al., 2017) was not considered.…”

Preface

“…Accordingly, Van der Deijl et al (2017) identified the factors controlling the sediment trapping of two newly created freshwater tidal wetlands under varying conditions of river discharge, tide, and wind in the Netherlands. A positive sediment budget and trapping efficiency was found where there was a continuous supply of river water and sediment, while sediment was lost where the wetland lies further from the river and had a lower sediment supply.…”

Effects of Sea‐Level Rise on Dredging and Dredged Estuary Morphology

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Estuaries are bodies of water with one or more open connections to the sea (Leuven et al, 2016) which develop at the land-sea interface due to delivery of sediment from both rivers and the coast (Nicholls et al, 2020). Estuaries that have developed naturally tend to have a converging planform shape, often with mutually evasive ebb and flood channels which create a multi-channel system (Jeuken & Wang, 2010;van Dijk et al, 2021;Weisscher et al, 2022). They have several intertidal shoals and bars, particularly at their widest points (Leuven et al, 2016), and extensive floodplains (van Veen et al, 2005).

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“…Dredging activities can be broadly split into two categories: (a) capital dredging events, which deepen channels permanently, and (b) maintenance dredging, which is the periodic process of keeping channels at a specific depth. Typically, systems undergo capital dredging every 10-20 years, while maintenance dredging typically occurs year-round, particularly in systems that are home to large ports (Cox, Dunn, et al, 2021;Cox, Huismans, et al, 2021;van Dijk et al, 2021). The dredged material can be either completely removed from the system (sold to market or dumped offshore) or reused and relocated within the system.…”